1. Field of the Invention
The present invention relates to a polymerization apparatus useful for polymerizing a monomer having an ethylenically unsaturated double bond, and a process of producing a polymer using the same.
2. Description of the Prior Art
In suspension polymerization of a monomer having an ethylenically unsaturated double bond such as, for example vinyl chloride in an aqueous medium, water, a monomer, a polymerization initiator, a dispersant, and optionally other various additives are normally charged through pipes into a polymerization vessel, after which the polymerization reaction is initiated. In order to maintain the reaction system at a prescribed temperature during the polymerization reaction, heat of polymerization is removed by passing cooling water through a water jacket and optionally, through a reflux condenser, a baffle and a coil provided for the vessel.
With the polymerization of vinyl chloride or the like, there is a problem in that polymer scale is liable to be deposited in the polymerization vessel. A conventional method of preventing the polymer scale deposition, involves forming on the inner wall of the polymerization vessel and other parts with which the monomer comes into contact during the polymerization, a coating film of a polymer scale preventive agent such as a dyestuff, a pigment, a water-soluble polymer, a nitrite, a nitrogen-containing aromatic compound, a heterocyclic compound, an iodine compound, a pyrogallol derivative, a phenolic compound or an aromatic amine compound. Such an agent is known from publications described later.
In recent years, from the viewpoint of labor hygiene and environmental safeguards, efforts have been made to reduce as much as possible the frequency of opening the polymerization vessel for internal inspection or cleaning. An effective process is thus required that can prevent the deposition and growth of polymer scale in the polymerization vessel, over a long-term operation involving the repetition of about 100 or more batches of polymerization runs.
However, if such a long-term operation as described above is carried out with only the coating of the polymer scale preventive agent, polymer scale is still able to grow on the inner wall of the polymerization vessel and the like. This causes a problem in that the polymer scale may peel off from the internal wall and mix with a polymeric product being produced, causing an increase in fish eyes in the resultant polymeric products and a reduction in the quality thereof.
In recent years, in order to improve productivity, polymerization vessels have been made larger in size. Consequently, the proportion of heat removed by the water jacket is reduced with an ever increasing amount of heat being removed by the reflux condenser. Thus, for a polymerization vessel having an internal volume of 40 m.sup.3 or more, and more particularly for one of 80 m.sup.3 or more, heat removal by the reflux condenser has become indispensable.
The reflux condenser is installed outside the polymerization vessel, and a monomer condensed in the condenser is returned to the polymerization vessel through a pipe (hereafter referred to as a liquid-return pipe) which opens at one end into the gaseous phase region inside the polymerization vessel. The end of the liquid-return pipe is worked so as to be virtually flush with the inner wall of the polymerization vessel, with the result that when the liquid monomer refluxed from the reflux condenser flows out the end of the liquid-return pipe, it normally attaches to the inner wall surface and flows down into the reaction mixture. In a polymerization vessel with an internal volume of 40 m.sup.3 or more, the volume of monomer which is condensed in the reflux condenser and refluxed to the polymerization vessel increases with an increase in the quantity of heat removed by the reflux condenser. In such a situation, even though a polymer scale preventive agent is coated on the inner wall surface of the polymerization vessel, it is difficult to prevent a build up in polymer scale on the surface, particularly in the area below the opening of the liquid-return pipe where a strip like deposit tends to form on the inner wall surface. Once polymer scale has deposited on the coating film of the polymer scale preventive agent, the polymer scale prevention effect thereof is impaired.
In addition to the liquid-return pipe from the reflux condenser, various other pipes including a pipe for changing raw materials and a pipe for recovering unreacted monomer gases are normally connected from the outside to open into the gaseous phase region inside the polymerization vessel. The monomer gas also enters into these pipes during polymerization, condensing into a liquid on the inner walls thereof, and flowing back from the pipe openings into the polymerization vessel. Since the pipe openings are similarly formed flush with the vessel wall as with the liquid-return pipe, polymer scale also tends to be deposited on the wall areas below the openings.